Engine cooling system (ECS) brush-commutated motors that drive radiator fans in automotive applications do not presently include effective protection for motor winding insulation and brushes from over-temperature operation during high-speed locked-rotor conditions. Recent automotive recalls for radiator fan motor fires have resulted in the development of specifications for motor protection at the ECS system and motor manufacturer level. Manufacturers prefer modified reset (SAE type II) locked-rotor protection which does not automatically reset after trip; protecting consumers from personal injury due to unexpected fan blade rotation and increasing locked-rotor event reliability at 150 A/15 Vdc and 300 A/24 Vdc conditions.
Any protector used for this purpose must be able to withstand underhood environmental conditions and preferably be adapted for mounting within the motor on the brush card in order to minimize salt spray degradation and optimize thermal sensitivity as well as to increase the motor manufacturer's value added.
So-called self-hold protectors in which electrical contacts of a protector are maintained in the open contacts position following actuation to interrupt current to a load due to an overload until power is removed from the protector are known. For example, in UK Patent Application GB 2252674A, published Aug. 12, 1992, a positive temperature coefficient of resistivity (PTC) heating element is received in a molded plastic housing that also mounts a snap-acting bimetal switch actuating element in heat conductive relation with the PTC element. Upon overheating of an appliance with which the protector is used, the snap-acting element actuates to open the contacts interrupting the load current and causing current to flow through the PTC element which self-heats to a high-resistance temperature thereby maintaining the snap-acting element in the actuated position with minimal current then passing through the PTC element. This device would not be useful in an automotive underhood environment, among other reasons, due to the fact that the various components are not sealed off from the environment and would be subject to deterioration from salt spray and the like.
Another example is shown in U.S. Pat. No. 6,020,807 that issued Feb. 1, 2000. The protector of this patent comprises a can shaped, electrically conductive metallic housing in which a bimetal blade is cantilever mounted on the bottom wall of the housing, the blade mounting a movable electrical contact on the free end thereof and movable into and out of engagement with a stationary contact mounted on an electrically conductive cover plate that is mounted on the housing through an insulating sheet. A PTC element is mounted between and in electrical engagement with the cover plate and the fixed end of the bimetal blade. The bimetal blade carries the load current and when actuated causes the contacts to open thereby interrupting the load current and the blade is then maintained in the actuated position by heat from the PTC element which then limits the load current to a trickle level. In this arrangement, heat conduction between the PTC element and the bimetal blade is optimized due to the minimized thermal resistance between the two components. As a result, the arrangement limits the size and power produced by the PTC component to significantly affect all boundary conditions, limiting the useful self-hold function range relative to ambient temperature and voltage.